1. Field of the Invention
The present invention relates to a semiconductor laser diode chip (an LD chip), and to a method for mounting this semiconductor laser diode chip onto a substrate. In particular, the present invention relates to an LD chip and its mounting method suitable for a passive alignment technique without performing optical coupling to an optical waveguide and an optical fiber by emitting light from the LD chip.
2. Description of the Related Art
An LD chip is, conventionally, optically coupled to an optical waveguide formed on a substrate, or to an optical fiber arranged on the substrate by monitoring intensity of light outputted from the optical fiber in an oscillating state of the LD chip, and performing a positioning operation such that the light intensity is maximized. However, it is necessary to flow an electric current through the LD chip to oscillate the LD chip. Further, productivity is low since it is also necessary to make a position adjustment while the light intensity outputted from the optical fiber is monitored. In particular, when the LD chip is optically coupled to the optical waveguide and an adjusting mechanism is arranged, the structure becomes complicated and it is difficult to monitor in advance light outputted from the optical waveguide.
Therefore, mainly in a module in which the LD chip and the optical waveguide are optically coupled to each other, the consideration of an assembly technique called a passive alignment mounting technique is forwarded instead of the above method to reduce the number of assembly works. For example, a construction described in U.S. Pat. No. 5,414,787 is known as a construction relative to positioning of the LD chip and its mounting method by the passive alignment technique. In the passive alignment technique, a coupling technique of the LD chip and the optical fiber is constructed by fixedly mounting a semiconductor laser and the optical fiber at respective predetermined positions, and it is not necessary to adjust an optical axis. Accordingly, it is not necessary to oscillate the LD chip in the passive alignment technique, either. However, in any technique, it is normally necessary to perform a mounting operation with sub-.mu.m accuracy to couple with sufficient efficiency the semiconductor laser and the optical fiber to each other.
In the LD chip used in the mounting method using the conventional passive alignment technique, a positioning mark is formed on a substrate-side face of the LD chip. This mark is normally formed of a metallized layer. In the mounting method of the LD chip using the passive alignment technique, it is necessary to accurately recognize the position of an active layer buried in a semiconductor laser body to mount the LD chip to the substrate with high accuracy.
No active layer can be normally confirmed directly by its outward appearance. Therefore, a positioning mark is formed on a surface of the LD chip body. The LD chip is positioned by confirming the positioning mark. Accordingly, relative position accuracy of the positioning mark to the active layer is very important in the passive alignment.
However, at a position facing the active layer, it is necessary to perform solder joining to a heat sink such as a silicon (Si) substrate in order to radiate heat. Accordingly, an Au metallized layer 2 for the solder joining is formed in the position facing the active layer. Therefore, it is difficult to pattern a mark requiring a certain wide area such as the positioning mark. Accordingly, in the conventional LD chip, the distance between the positioning mark and the active layer is as far as 50 .mu.m to 100 .mu.m.
A method of directly measuring position accuracy by observing a section of the LD chip body is most accurate to confirm the relative positions of the active layer of the LD chip and the positioning mark of the LD chip. However, in the conventional LD chip, since the distance between the positioning mark and the active layer is 50 .mu.m to 100 .mu.m, a measurement error in measuring the relative position accuracy of the positioning mark to the active layer is as large as 2 to 3 .mu.m, or more. Therefore, no sufficient recognizing accuracy can be obtained with respect to mounting using the passive alignment technique requiring sub-.mu.m accuracy.